Trialkylammonium tetrakis(pentafluorophenyl) borate and (aryl)dialkylammonium tetrakis(pentafluorophenyl) borate are used as co-catalysts for a polymerization reaction using a metallocene catalyst. Examples of known methods for producing the borates include the following methods (i) and (ii) (refer to Patent Literature 1).
(i) Method for producing di(tallow-alkyl)methylammonium tetrakis(pentafluorophenyl) borate bromo magnesium tetrakis(pentafluorophenyl) borate→magnesium di[tetrakis(pentafluorophenyl) borate] magnesium di[tetrakis(pentafluorophenyl) borate]+di(tallow-alkyl)methylamine+hydrogen chloride→di(tallow-alkyl)ethylammonium tetrakis(pentafluorophenyl) borate
(ii) Method for producing dimethylanilinium tetrakis(pentafluorophenyl) borate potassium tetrakis(pentafluorophenyl) borate+N,N-dimethylaniline+hydrogen chloride→dimethylanilinium tetrakis(pentafluorophenyl) borate